Processes such as enhanced oil recovery require large quantities of elevated pressure nitrogen which is produced by the cryogenic separation of air. The efficient production of refrigeration for such air separation systems is particularly important, especially if some liquid product is also desired. Moreover, fluctuating demand for liquid product increases the need for an effective refrigeration generation arrangement.
In systems that employ feed air expansion, increased liquid demand significantly diminishes product recoveries, necessitating large increases in power consumption and requiring unreasonable range-ability of feed compressors. For double column nitrogen plants, the large drop in nitrogen recovery also leads to a large change in column pressures. This also results in poor plant operability. In systems that employ waste expansion, increased liquid demand necessitates significant increases in column pressures, which hurts product recoveries and greatly increases the plant's power consumption. The changing column pressures negatively affect plant operability. Systems that employ conventional modes of product nitrogen expansion fall short in one of two ways: (1) Increased liquid demand requires unreasonable rangeability of the lower stages of the product compressor, or (2) If designed to properly accommodate variable liquid demand, existing product expansion methods require multiple turbines and operate at high pressures that severely limit heat exchanger core choices.
Accordingly, it is an object of this invention to provide an improved system for producing elevated pressure nitrogen by the cryogenic separation of air.